Nuclear fuel element incorporating helical fissionable wire



Sept. 27, 1966 c. H. BLOOMSTER ETAL NUCLEAR FUEL ELEMENT INCORPORATING HELICAL FISSIONABLE WIRE BE; FEE- United States Patent f 3,275,525 NUCLEAR FUEL ELEMENT INCORPORATING HELlCAL FISSIONABLE WIRE Clarence H. Bloomster and Joseph J. Hauth, Richland, Wash., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 20, 1965, Sen No. 473,538 6 Claims. (Cl. 176-73) The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

This invention relates to a heterogeneously enriched fuel element in which the concentration of fissile atom is variable. More specifically, this invention relates to a fuel element of the aforementioned type which is plutonium bearing.

Mills et al. application S.N. 361,630, filed Apr. 20, 1964, now Patent No. 3,230,151, discloses and claims a fuel element in which a highly enriched wire extends longitudinally through ceramic fuel of low enrichment. For various reasons having to do with reactor operation, it may be desirable for the concentration of fissile atom in such a fuel element to vary longitudinally and/or radially. Yet difiiculties are encountered in making a fuel element of this description.

An object of the present invention is to provide a fuel element with a variable concentration of fissile atom. This fuel element is relatively cheap and easy to manufacture.

We have invented a fuel element in which a highly enriched wire is specially shaped so as to provide a variation in the concentration of fissle atom. According to the invention, the wire is shaped as a helix of varying pitch. Further according to the invention the wire may be a helix of varying diameter.

In the drawings:

FIG. 1 is a longitudinal sectional view of a fuel element having a helical wire of varying pitch extending through a stack of fuel pellets;

FIG. '2 is a longitudinal sectional view of another fuel element having the helical wire of varying pitch extending through vibrationally compacted crushed fuel; and

FIG. 3 is a longitudinal sectional view of a fuel element provided with a helical wire varying in diameter.

As shown in FIG. 1, a fuel element 10 is provided with a plurality of annular sintered natural-uranium-dioxide pellets L1 stacked so as to have an axial opening 12 extending therethrough. The pellets 11 are enclosed in a Zircaloy-2 jacket 13 closed by end caps 14. Within the pellets 1-1 is a helical wire 15 of plutonium-containing material. The helical wire 15 extends through the axial opening 12 in the stack of pellets 11 and varies in pitch from a maximum at a mid region of its length to a minimum at its ends. 'Both the wire diameter and the helix diameter are uniform. The ends of the wire 15 may be joined to end caps 14. The void space in the axial opening 12 not occupied by the wire 15 is filled with mass 16 of granular natural-uranium dioxide (20 mesh). The wire 15 may be of plutonium-15% zirconium alloy and may be bare or clad in a suitable protective material. It should be understood that in the fuel element of FIG. 1 as Well as those of the other figures the wire may contain not only plutonium-239, but uranium-233 or uranium- 235 instead.

Of the various methods that can be used to produce a continuously varying pitch in a metallic helix, the simplest involves suspending the helix, with a weight attached to it, and allowing the helix to stretch to the desired length. The weight on each increment of the helix increases as the distance from the point of suspension de- 3,275,525 Patented Sept. 27, 1966 ICC creases, with a resulting change in pitch which is a function of helix composition, helix diameter, and wire diameter. in the application of this method to the wire 15 of FIG. 1, the point of suspension will have to be a mid region of the length of the wire, and the suspension operation will have to be carried out twice, once for each half length of the helix.

The decrease in pitch from the center to both ends of the helical wire 15 brings axial variation in enrichment, the advantages of which are obvious to those skilled in the art. The helical wire 15 brings such variation in enrichment to a fuel element of the type claimed in the aforesaid Mills et al. application.

As shown in FIG. 2 a fuel element 17 comprises the Zircaloy-2 jacket '13, end caps 14, and the plutoniumcontaining wire 15. The fuel element 17 also comprises a mass 18 of crushed vibrationally compacted naturaluranium dioxide. The vibrational compaction may be carried out as disclosed in Hauth Patent 3,042,594, dated July 3, 1964. The ends of the wire 15 are fastened to the end caps 14 so as to be held in tension.

As is the case with the wire 15 of the fuel element 10 of FIG. 1, the wire 15 of the fuel element 17 of FIG. 2 has a relatively small diameter of helix and is well spaced radially from the interior of the jacket 1.3. 'It is Within the scope of the present invention to increase the diameter of helix so that the wire 15 engages the interior of the jacket 15. Such an arrangement is preferable in a compact power reactor, where the maximum heat flux per unit of area is desired.

As shown in 'FIG. 3, a fuel element 19 comprises the jacket 13, end caps 14, and a mass 18 of crushed vibrationally compacted natural-uranium dioxide. The fuel element 19 also includes a helical plutonium-containing wire 20 whose diameter of helix varies from a minimum at a mid region to a maximum at the ends. This arrangement brings a radial variation in the concentration of the fissile atom and tends to flatten the flux distribution along the fuel element 19. If the Wire 20 has uniform pitch, the amount of wire per unit of length of the fuel element 19 varies from a minimum at the center to a maximum at the ends, and so there is a corresponding variation in the concentration of the fissle atom longitudinally of the fuel element. If the fuel element 19 is not to have a longitudinally variable concentration of fissile atom, then the pitch of the wire 20 must vary from a minimum at the center to a maximum at the ends in the manner shown in FIG. 3, so that the amount of wire per unit of length is constant.

In another form of the invention involving a cluster of rod-type fuel elements, the heat generation can be made optimum by enriching different fuel elements comprising the assembly with different amounts of plutonium. This is accomplished simply and economically by forming a helix of constant pitch and diameter, and using different lengths of helix (corresponding to the different amounts of plutonium required) which are stretched to the same final length, with or Without a continuous pitch variation in the final product. This simple method of enriching fuel elements to varying degrees would facilitate assembly of balanced reactor core loadings.

Still another form of the present invention may involve the application of a helical plutonium-containing wire of varying pitch to the exterior of the fuel-element jacket. Such a wire may serve to provide initial enrichment of a fuel element or to re-enrich or rejuvenate the fuel element after radiation. The helical wire might be formed of a stainless-steel or Zircaloy-2 tube of small diameter, a plutonium-containing wire within the tube, and a filler of magnesium oxide. The externally applied helical wire may serve to space the fuel element to which it is applied from other fuel elements. If the external wire is applied for rejuvenation of the fuel element, there is a separation of the plutonium used for rejuvenation from that originally applied to the interior of the fuel element Whether straight or helical.

In all forms of the present invention the fuel element is relatively cheap and easy to manufacture, because the enriched wire thereof, being of uniform diameter, is low in cost and is easily bent into a helix.

It is understood that the invention is not to be limited by the details given herein but that it may be modified Within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

We claim:

1. A nuclear-fuel element comprising a long mass of ceramic fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy and a varying helical wire of a metallic fuel containing a relatively high amount of a material fissionable by neutrons of thermal energy extending axially of the mass, wherein the varying helical wire is a helical wire of pitch varying from a maximum at a mid region to a minimum at the ends of the wire.

2. A nuclear-fuel element comprising a long mass of ceramic fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy and a varying helical wire of metallic fuel containing a relatively high amount of a material fissionable by neutrons of thermal energy extending axially of the mass, wherein the varying helical wire is a helical wire of a diameter of helix varying from a minimum at a mid region in the length of the Wire to a maximum at the ends of the wire.

3. A nuclear-fuel element comprising a stack of annular sintered pellets of fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy, and a helical wire of metallic fuel containing a relatively high amount of a material fissionable by neutrons of thermal energy, extending through the stack of pellets, the helical pitch of the wire varying from a maximum at a mid region in the length of the wire to a minimum at the ends of the wire.

4. The nuclear-fuel element specified in claim 3 and further comprising a mass of vibrationally compacted fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy extending for the length of the stack of pellets through the pellets and over the helical wire.

5. A nuclear-fuel element comprising a long mass of vibrationally compacted crushed fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy, and a helical wire of metallic fuel containing a relatively high amount of a material fissionable by neutrons of thermal energy extending axially through the mass, the helical pitch of the wire varying from a maximum at a mid region in the length of the wire to a minimum at the ends of the wire.

'6. A nuclear-fuel element comprising a long mass of vibrationally compacted crushed fuel containing a relatively low amount of a material fissionable by neutrons of thermal energy, and a helical wire of metallic fuel containing a relatively high amount of a material fissionable by neutrons of thermal energy extending axially through the mass, the diameter of helix of the wire varying from a minimum at a mid region in the length of the wire to a maximum at its ends.

References Cited by the Examiner UNITED STATES PATENTS 2,879,216 3/1959 Hurwitz et al. 176-90 X 3,043,761 7/ 1962 Reynolds 17690 X 3,230,151 l/1966 Mills et al. 176-73 References Cited by the Applicant UNITED STATES PATENTS 2,947,080 8/1960 Kates et al. 2,968,601 1/1961 Evans et al.

L. DEWAYNE RUTLEDGE, Acting Primary Examiner. 

1. A NUCLEAR-FUEL ELEMENT COMPRISING A LONG MASS OF CERAMIC FUEL CONTAINING A RELATIVELY LOW AMOUNG OF A MATERIAL FISSIONABLE BY NEUTRONS OF THERMAL ENERGY AND A VARYING HELICAL WIRE OF A METALLIC FUEL CONTAINING A RELATIVELY HIGH AMOUNT OF A MATERIAL FISSIONABLE BY NEUTRONS OF THERMAL ENERGY EXTENDING AXIALLY OF THE MASS, WHEREIN THE VARYING HELICAL WIRE IS A HELICAL WIRE OF PITCH VARYING FROM A MAXIMUM AT A MID REGION TO A MINIMUM AT THE ENDS OF THE WIRE.
 2. A NUCLEAR-FUEL ELEMENT COMPRISING A LONG MASS OF CERAMIC FUEL CONTAINING A RELATIVELY LOW AMOUNT OF A MATERIAL FISSIONABLE BY NEUTRONS OF THERMAL ENERGY AND A VARYING HELICAL WIRE OF METALLIC FUEL CONTAINING A RELATIVELY HIGH AMOUNT OF A MATERIAL FISSIONABLE BY NEUTRONS OF THERMAL ENERGY EXTENDING AXIALLY OF THE MASS, WHEREIN THE VARYING HELICAL WIRE IS A HELICAL WIRE OF A DIAMETER OF HELIX VARYING FROM A MINIMUM AT A MID REGION IN THE LENGTH OF THE WIRE TO A MAXIMUM AT THE ENDS OF THE WIRE. 